Integration of solar collector system into existing hot water distribution system

Solar water heating systems maximize solar heat production when installed in a preheat configuration, i.e., the cold water supply is redirected to the solar storage tank where it can be preheated, and on a draw, the preheated water is directed into the backup or boiler tank for a final bump to temperature before servicing the usage. Of course this requires operation of the boiler equipment adding to the annual fuel use. Also, the firing rates in this mode may not be in the most efficient firing range of boiler operation.

One method to minimize the back-up boiler operation is to heat the DHW entirely by the solar collectors. One method to accomplish is to recirculate the water leaving the collector again through the collectors if it is not hot enough. This set-up requires a three-way valve between the return and either solar storage tank. This will allow the heat transfer fluid warmed by the collector to be directed either to the storage tank or to the line going back to the solar collectors. The control logic is when the solar tank temperature is higher than the return of the recirculation water; the solar collectors can heat it up further. When the solar tank temperature is below the temperature of the heat transfer fluid leaving the collector, it is directed to the tank.